Abstract--A new control strategy is proposed for power quality enhancement in the three phase three wire system. A fuzzy logic controller is integrated in Synchronous Reference Frame (SRF) topology to minimize the transient time of Active filter. The proposed algorithm evaluates energy of capacitor rather than voltage and analyzes a fast response. A comparison has been made to analyze the performance in transient time using conventional and proposed controller when there is abrupt fall in load. The model is developed using MATLAB / "SimPower Systems” Block library.

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Index Terms--Power Quality, Active Power Filter, Fuzzy Logic, THD Reduction, Transient Time

I. NOMENCLATURE Synchronous Reference Frame (SRF),Active Power Filter

(APF),Fuzzy Logic (FL)

II. INTRODUCTION

HE severity of harmonic pollution in power networks is increasing extensively. Power electronic devices capture the attention of power system engineers to develop

dynamic and adjustable solutions to the power quality problems. A solution is known as Power Filter .Conventionally passive filters are mainly applied for harmonic restrain and reactive power compensation due to its advantages of less cost, simple structure, robust stability and easy for maintenance. But passive filters have large dimension, their performance is affected by system and it is possible for passive filters to have resonance with system. After all, it is not enough to manage harmonic only by using passive filters. We must use new technology active power filter for management of power quality in power distribution system. APF consist of Voltage-source inverters and a DC capacitor; have the ability to adjust the amplitude of the synthesized ac voltage of the inverters by means of pulse width modulation or by control of the dc-link voltage, thus drawing either leading or lagging reactive power from the supply. [1]-[5].The

Sakshi Bangia is with Department of Electrical Engineering, YMCA Universiy of Science and Technology, Faridabad (e-mail: sakshibangia@gmail.com).

P.R.Sharma is Chairman, Department of Electrical Engineering, YMCA Universiy of Science and Technology,Faridabad(e-mail: prsharma1966@gmail.com). Maneesha Garg is with Department of Humanities and Applied Science,YMCA University of Science and Technology,Faridabad (e-mail:

garg_maneesha@yahoo.com)

compensation current/voltage reference signal estimates the information regarding the harmonic currents and other system variables and drives the overall system controller. This in turn provides the control for the switching signal generator. The output of the switching signal generator controls the power circuit via a suitable interface. Thus the power circuit can be connected in parallel, series or parallel/series configurations depending on the interfacing inductor/transformer used.

Fig 1. Shunt Active Power Filter

III. PROBLEM FORMULATION Fig 1 shows the basic components of shunt Active Filter. Shunt APF acts as a current source, compensating the harmonic currents due to nonlinear loads.The operation of shunt APF is based on injection of compensation current which is equals to the distorted current, thus eliminating the original distorted current. This is achieved by “shaping” the compensation current waveform )(i f , using the VSI

switches. The shape of compensation current is obtained by measuring the load current (il ) and subtracting it from asinusoidal reference. [6].The aim of shunt APF is to obtain a sinusoidal source current is using the relationship:

iii fls −= (1)

The nonlinear load current can be written as the sum of the fundamental current component (ilf ) and the current

harmonics (ilh ) according to

iii lhlfl += (2)

then the injected compensation current by the shunt APF should be

Fuzzy Logic Based Reduction in the Response Time of Active Filter

Sakshi Bangia, P.R.Sharma ,Maneesha Garg

T

2012 2nd International Conference on Power, Control and Embedded Systems

978-1-4673-1049-9/12/$31.00 ©2012 IEEE

ii lhf = (3)

the resulting source current is

iii fls −=

=ilf (4)

IV ENERGY BASED REFERENCE CURRENT GENERATION

The reactive and harmonic compensation is carried by injecting appropriate currents into the circuit through a compensator i.e., shunt active filter. Figure 2 shows the conventional Synchronous Reference Frame block diagram of the control strategy for Active Filters. The dc voltage, load current, and PCC voltage are sensed. A PI controller is realized over the reference values and sensed of DC voltage of active filter. The other PI controller is realized over the reference values and sense of ac voltage at PCC. The references currents generated are fed to hysteresis controller, which is used for tracking control. The dc voltage control loop is given by

∫+= edte kkp idcpdcdc (5)

Where k pdc and kidc are the proportional and integral

gains respectively for the dc voltage control loop and e is the error between the reference dc voltage and the average dc voltage.

In this paper a new control strategy based on energy is implemented using fuzzy logic control in place PI control. The energy demanded by the capacitor to charge from actual voltage to the reference value can be computed as

)(21 22 vvcw dcdcrefdcdc −= (6)

Now the total dc power required by the capacitor is computed as follows:

)()( 2222 vvkvvkp dcdcrefiedcdcrefpedc−+−= ∫ (7)

Fig 2. Synchronous Reference Frame control strategy

Design of proportional and integral gain in the energy based controller can be determined by solving the equation

(3) and (6) by assuming vv dcdcref + is equal to

vdcref2 Hence it can concluded that ratio of gains of

proportional to integral controller is many times larger in case of new scheme introduced here.[7]

The Hysteresis current controllers are accomplished in the three phases independently. Each current controller determines the switching signals to the inverter. The error signal reference and actual source current are calculated and compared within a small hysteresis band hb. The switching logic for phase a is formulated as below.

If ifa < ( i*fa -hb) upper switch of VSC turned OFF and lower switch is ON

If i fa < ( i*fa +hb) upper switch of VSC is turned ON and lower switch is OFF

In the same fashion, the switching of phase b and c devices are derived.

IV PROPOSED FUZZY LOGIC CONTROLLER

Fuzzy Logic approach represents a control methodology, in such as a way of processing data by allowing partial set membership rather than crisp set membership or non-membership. FL incorporates a simple, rule-based IF X AND Y THEN Z approach to a solving control problem. A Fuzzy controller composed of stages viz fuzzification, knowledge base, and inference mechanism. The knowledge base is composed of data base and a rule base and is designed to obtain good dynamic response under ambiguity in process parameters. The data base consists of input and output membership functions. The inference mechanism uses a ensemble of linguistic rules to covert the input condition in to a fuzzified output. Finally defuzzification is used to convert the fuzzy output in to control signals.

In the proposed energy based control strategy, input to fuzzy logic controller is the error between the squared dc reference voltage and square of the sensed PCC voltage.

Fig 3. membership functions for input and output variables

In this case, three fuzzy subsets viz SN (Negative Small), (Zero), and SP (Positive Small) have been chosen. Membership functions used for the input and output variables used here are shown in Fig.3. As both inputs have three

subsets, a fuzzy rule base formulated for the present application is given in Table 1.

TABLE 1

Fuzzy Rules change of

error

error

SN S SP

SN PS Z SN

S Z PS Z

SP SN Z PS

V SYSTEM CONFIGURATION AND SIMULATION RESULTS

This section presents the detail description of the simulation carried out for three phase three wire system using MATLAB with its simulink and Power System Block set toolbox. The system parameters are given in Appendix1 The ac load consists of a three phase unbalanced load and a non linear load of bridge rectifier (three phase) feeding a highly inductive R-L load. The model is analyzed for switching of load to half of its value from 0.4s to 0.8s. Fig 4 shows simulink model for the proposed system consisting of power supply, load and active filter connected through ripple filter. Thus in order to evaluate the performance of an active power filter, different aspects is considered. Load currents, dc voltage and source voltage are sensed to calculate the reference voltage. The switching signal is derived using Hysteresis controller.

Fig 4. Simulink model of the implemented system

The performance of active filter is demonstrated to minimize the transient time and power factor correction with harmonic reduction. The model is analyzed under switching of load from full load to half load at 0.4s to 0.8s.

V PERFORMANCE OF RESPONSE TIME OF ACTIVE FILTER

The performance of dc voltage of active filter is analyzed using MATLAB/SIM POWER SYSTEM BLOCKS. The Synchronous Reference Frame control strategy is implemented with same load parameters, supply voltage other system parameters .The transient performance of active filter are implemented using conventional method and the proposed energy based controller. Comparison of PI controller and fuzzy controller are made in implementing conventional and energy based control strategy.

A. Transient Performance by PI controller Method

The performance of active filter during non-linear load unbalanced load condition is studied by making abrupt changes in the ac load supplied by the ac load bus. The performance of conventional control method using PI controller is as shown in fig.5. In the simulation study, the load is halved at the instant t= 0.4 s and brought back to full load at t=0.8s. The shunt connected active filter works in such a way that when capacitor voltage rises above the reference value due to abrupt fall in load, the capacitor absorbs excess power from the source. Based on the values of PI controller gains, controller will be brought back the capacitor voltage to the reference value within a few cycles. Similarly, when the load is brought back to the full load at instant 0.8 s, the dc capacitor supplies power to the load , hence, the dc voltage falls below the reference value. The dc capacitor supplies power to the load transiently. Due to the PI controller action, the capacitor voltage will gradually build up and reach its reference value. [7]

Fig 5. Transient time of Vdc using conventional method and PI controller

Fig 5 shows the response time taken by the dc voltage, starts from 0.4s at the instant when load gets to half and it gets stable at 0.415s.Similar observations can be found at the instant when at t=0.8 halved load gets change to full load.

Fig 6 shows the transient performance of the Energy based voltage controllers using PI controller. Clearly time taken by the energy based method is from 0.4s to 0.412s

Fig 6. Transient time of Vdc using Energy Based method and PI controller

B Transient Performance by Fuzzy logic controller

To introduce the concept of linguistic and vague computation techniques, the response time is also evaluated using Fuzzy Logic Controller. Fig 7 shows the response time of dc voltage when load gets halved at 0.4s using FLC. It can be clearly analyzed that time taken by dc voltage to get stable is from 0.4s to 0.42s

Fig 7. Transient time of Vdc using conventional method and fuzzy controller

Thus the response time taken by the dc voltage using fuzzy logic controller starts from 0.4s at the instant when load gets to half and it gets stable at 0.42s.Similar observations can be found at the instant when at t=0.8 halved load gets change to full load.

Fig 8 shows the transient performance of the Energy based voltage controllers using PI controller. Clearly time taken by the energy based method is from 0.4s to 0.405s

Fig 8. Transient time of Vdc using Energy Based method and fuzzy controller

The comparison of the transient time of dc voltage when there is sudden decrease in load by various methods has been formulated in TABLE II. It can be clearly analyzed that Energy Based method with fuzzy controller is prove to be the best among all.

TABLE II

Comparison of Transient Time Controller

method PI FUZZY

CONTROLLER

Conventional method

0.015 s 0.02 s

Energy Based method

0.012 s 0.005 s

B Dynamic Performance of active filter using Fuzzy logic controller

The dynamic performance of active filter using Synchronous Reference Frame strategy but with fuzzy controller is shown in Fig 9 .The supply voltage, supply current, load voltage, load current Vdc voltage and active filter current are demonstrated. It has been observed that THD of load current is 17.49% while it has been reduced to 3.62% of supply current. The dc-voltage is getting constant to 550V.

The FFT analysis of the supply current phase a is as shown in the Fig 10 showing THD as 3.62%.

Fig 9. Dynamic performance of active filter with non-linear unbalanced load for harmonic compensation The supply voltage, supply current, load voltage, load current Vdc voltage and active filter current are demonstrated.

Fig 10. FFT analysis of source current

VI CONCLUSION

A new three phase three -wire, energy based controlled strategy implemented with fuzzy logic controller is proposed to reduce the response time of dc link voltage. A great reduction in the transient time has been noticed with fuzzy logic controller. Also the dynamic performance of active filter has been analyzed which meet IEEE 519-1992 standard recommendations of harmonic levels.

Appendix 1 Supply voltage =415V (L-L), 50 Hz Loads: (i) Linear: phase a= 25Ω Phase b=44 Ω and 81mH Phase c =44 Ω and 81 mH (ii) Non linear: Three phase full bridge rectifier drawing 5A

Shunt Active Filter: Interfacing Inductance, Lf = 5.5 mH, 0.25 Ω DC bus capacitance: 22000e-6F. Referenced Dc voltage: 550 V

IV. REFERENCES

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